As far as birthday parties go, it was a good one. Billy, 10, and his five best friends were helicoptered out of school in west London and dropped at a secret location for a meeting with a corrupt politician, which led to a breakneck powerboat chase down the Thames. Then there was a kidnapping, and a car chase through the city, and an ambushing … and then Bond – sorry, Billy – and his friends were chauffeured back home for a sleepover. Too much? Blame Sharky & George, the London-based party planners who fulfil the far-fetched fantasies of the privileged children of oligarchs and celebrities.

Unsurprisingly, every child in London wants a Sharky & George party. Musical chairs and coloured bunting from the supermarket seem rather lame compared with a recent Sharky & George camp-out for a nine-year-old that turned into a full-blown “SAS night mission” to free a hostage using laser guns. The actor Ewan McGregor was the first big name to book them in 2008, and they’ve since entertained the offspring of Elizabeth Hurley, David Cameron and Bear Grylls, as well as youngsters at Paul McCartney’s wedding to Nancy Shevell in 2011.

But founders Charlie Astor and George Whitefield, both 30, who met at Eton, insist that their most popular parties are much more low key. “Mothers are surprised when our entertainers turn up on bicycles with a small rucksack,” Whitefield explains. “They’re expecting a couple of vans.”

No wonder, given that they will have forked out at least £350 to write the words Sharky & George on their invitations. But the essence of a good party is not about expensive props, insists Astor. “We encourage children to use their imaginations. All we basically need is a rope, some rocket balloons, a parachute and some sweets.”

The formula is a winning one. After an afternoon of “dance-off pass the parcel”, parents are usually left wondering if hiring Sharky & George every weekend would be acceptable.

The twosome insist – less than convincingly – that they are pranksters first, businessmen second. It was a practical joke played on their housemaster, Mr Roberts, that inspired them to become children’s entertainers in the first place. “We encouraged him to play a game that involved securing a funnel in the front of his trousers and dropping coins into it from his forehead – no hands,” Astor says. “While he attempted to catch his first coin, George poured a jug of water into the funnel.” Roberts’s punishment for them was inspired: they were to entertain guests at his daughter’s sixth birthday party the next weekend.

Astor and Whitefield, who were 16 at the time, were horrified by the prospect. “We were told to be ourselves, no weirdness or silly voices, which is quite difficult when you’re not used to being around children,” Whitefield says.

They stuck to the games they had played when they were young – grandmother’s footsteps, musical bumps – and received a hero’s send-off at the end. “It gave us an idea,” says Astor. “When we went on to Bristol University we threw a load of parties for George’s nephews and nieces and were soon being booked by their friends’ parents.”

By the time they graduated, they were organising several parties a month under the name Sharky & George (after the children’s television cartoon), and continued this as a sideline when they moved to London to start “serious” jobs, Astor in sales and marketing at Volkswagen, and Whitefield as a financial headhunter for Armstrong International. But combining careers with being party planners proved too exhausting, and in 2007 they announced to their parents that they were becoming clowns full-time. “George’s dad is a chocolate maker, so he was quite up for it, but my parents had been quite happy that I had a nice, safe job,” says Astor.

There was a lot of larking about initially. But then they got serious: now Sharky & George organises 50 parties a week from offices in Battersea in south-west London, and employs six full-time staff and 40 weekend entertainers.

So what makes a child who has everything want a Sharky & George party? According to Astor, the children regard them as “older cousin types” rather than adults. For a party of 30 children, there will be two entertainers, one to run the games and another to control the mood. “There’s usually a shy child, a boisterous one, a tearful one, so you need someone to calm people down, or cheer them up,” adds Astor, who has a girlfriend, Lindsay, while Whitefield and his wife Rosie have a six-month-old son, Bertie.

Unlike a lot of children’s entertainment, the emphasis is on playing games rather than watching a performance. “It’s organised mayhem. We’ll have three or four children firing water bombs from a catapult,” says Whitefield, who is single. They’ve altered traditional party games to make them more inclusive: at the end of “musical yoga mats”, for example, all 30 children will be crammed on to one mat together. “Some of the traditional party games are a bit strict,” says Astor. “At the age of five, you don’t want to get knocked out.”

The company has now expanded into luxury family holidays and Jack Wills-style kids’ clothes, and the duo have just written their first book, Don’t You Dare, out this week. Recognising that for some parents, 'budget’ is not an issue they have also ventured into bespoke “immersive” parties: film-making with the latest digital equipment; spy parties with tiny buttonhole cameras. The birthday girl at a recent party was taken by Cadillac to a recording studio to take part in her own X Factor-style competition (one of her friends played Simon Cowell in high-waisted jeans), while a survival party involved taking Bear Grylls’s children and their friends into the wilderness for a night. “I was showing them how to light a fire, and one of Bear’s boys told me, 'To be honest, it’s better to do it like this.’ ”

Parents are very much not invited to a Sharky & George party. If they get too close, they’re likely to find themselves at the receiving end of a water bomb. But according to Astor, the entertainers win over the adults in other ways – by handing around sandwiches at tea and making sure everyone is included in the conversation. They’ll even stay on at the end to fold up the bunting.

Astor and Whitefield insist that behind the frivolity, their company is serious. This is why they have set up The Exploration Society, which takes pupils from Thomas’s, Eaton House and Francis Holland on adventures. They’re hoping for government funding to give state-educated children similar experiences. “It’s learning by stealth,” Whitefield says. “We take children around London doing a massive treasure hunt, but they’re actually absorbing a lot of information.”

Their new book is the product of their own childish imaginations rather than anything overtly educational. There are pages of stickers, pranks to be performed on parents, and information on building dams and “dribble” sandcastles. “There was a large section about fart lighting which had to be removed for fear of burnt bottoms,” Astor says.

Their dream is that Sharky & George will become a “movement” to inspire children to use their imaginations. “We want to be the Jamie Olivers of fun,” says Astor.

And with a possible television show on the horizon, it looks as if they’re going to have the last laugh.

'Don’t You Dare' by Sharky & George (Egmont, £12.99, published June 3) is available to pre-order from Telegraph Books (0844 871 1514) at £11.99 + £1.35p&p.For more information on Sharky & George, visit sharkyandgeorge.com